Connecting rods which connect piston pins and crankpins are widely employed in engines for vehicles. A connecting rod has a larger end connected to the crankpin and a smaller end connected to the piston pin. For manufacturing the connecting rod, it is customary to integrally form a connecting rod from the larger end to the smaller end by forging or the like, for example, and then cracking the larger end into a cap part and a rod part.
An apparatus for cracking a connecting rod disclosed in Japanese Laid-Open Patent Publication No. 10-277848, for example, comprises an internal pressure applying device for applying an outward internal pressure to a bearing hole in the bearing of a connecting rod and a pair of external pressure applying devices for applying an external pressure to the bearing. The external pressure applying devices apply an external pressure to the bearing and the internal pressure applying device applies an internal pressure to the bearing. While the internal pressure applied by the internal pressure applying device is of a level capable of cracking the bearing, the bearing is instantaneously released from the external pressure applied by the external pressure applying devices, thereby enabling the internal pressure to instantaneously crack the bearing.
Japanese Laid-Open Patent Publication No. 2002-066998 discloses an apparatus for fracturing a connecting rod, comprising first and second support members for horizontally supporting a connecting rod, the first and second support members being disposed on a base of a pallet for placing the connecting rod thereon and movable in directions away from each other, a split mandrel having mandrel halves vertically mounted on the first and second support members and having respective outer circumferential surfaces for abutment against respective inner surfaces of an opening part, a wedge having tapered surfaces abutting against confronting end surfaces of the mandrel halves, for uniformly separating and spreading the mandrel halves away from each other, an actuator for applying a load to the wedge, and a control circuit for applying an initial load to the actuator to bring the mandrel halves into abutment against the respective inner surfaces of the opening part and thereafter applying a fracture load to instantaneously fracture the opening part.
Japanese Laid-Open Patent Publication No. 06-042527, for example, reveals an apparatus for cracking a connecting rod by equally applying breaking forces in directions to separate a pair of semicylindrical protrusions away from each other, to axially opposite sides of the semicylindrical protrusions which are fitted in a hole defined in a larger end of a connecting rod, thereby preventing the hole in the cracked larger end from being distorted.
In the cracking apparatus disclosed in Japanese Laid-Open Patent Publication No. 10-277848, Japanese Laid-Open Patent Publication No. 2002-066998, and Japanese Laid-Open Patent Publication No. 06-042527, the mandrel which comprises the mandrel halves in the form of identically shaped semicylindrical protrusions is inserted in the hole in the larger end of the connecting rod to position the connecting rod, and thereafter the wedge is pushed into holes of rectangular cross section defined in the mandrel halves to spread the mandrel halves away from each other, thereby cracking the larger end into the cap part and the rod part.
Stated otherwise, in the cracking apparatus disclosed in Japanese Laid-Open Patent Publication No. 10-277848, Japanese Laid-Open Patent Publication No. 2002-066998, and Japanese Laid-Open Patent Publication No. 06-042527, the mandrel halves that have identical areas of contact are placed in the hole in the larger end of the connecting rod, and the wedge is pushed in between the mandrel halves to spread the mandrel halves away from each other, thereby cracking the larger end contacted by the mandrel halves into the cap part and the rod part.
Some connecting rods for use in different engine types have a pair of bolt holes defined in respective side walls of the large end substantially parallel to the axis of the rod part for receiving therein respective bolts for integrally coupling the cap part and the rod part to each other, the bolt holes having respective central axes displaced off the centers of the fractured surfaces radially inwardly toward a joint hole in the larger end of the connecting rod, i.e., displaced closely to the joint hole in the larger end of the connecting rod.
When the larger end of a connecting rod for use in such a different engine type is cracked into a cap part and a rod part by moving apart the mandrel halves from each other, which have identical areas of contact with the inner circumferential surfaces of the joint hole in the larger end of the connecting rod, of the cracking apparatus disclosed in Japanese Laid-Open Patent Publication No. 10-277848, Japanese Laid-Open Patent Publication No. 2002-066998, and Japanese Laid-Open Patent Publication No. 06-042527, as shown in FIG. 23 of the accompanying drawings, crack lines 1 travel horizontally from the inner circumferential surface of a joint hole 3 in a larger end 2 to respective bolt holes 4, and then travel across and from the bolt holes 4 upwardly toward a cap part 5 away from a rod part 6. The crack lines 1 thus extend obliquely upwardly in directions away from the joint hole 3. As a consequence, irregular fractured surfaces are developed between the cap part 5 and the rod part 6, resulting in a defective product.